Method and arrangement for controlling cell change and a terminal of a cellular system

ABSTRACT

The invention relates to an improved method and arrangement for controlling a cell change performed by a terminal of a cellular radio system. In particular the invention relates to a cell change in a situation where the terminal is connected to a special service provided by its current cell, e.g. to the GPRS service (General Packet Radio Service). An essential idea of the invention is that in conjunction with a cell change order a base station sends to the terminal information about whether the terminal is allowed not to carry out the cell change order if the terminal does not know the timing information of the new cell assigned to it. This information is advantageously sent in a signalling message of a certain service, such as the GPRS service. It is an advantage of the invention that a terminal connected to a certain service will not make unnecessary cell change attempts which are doomed to fail. The invention helps direct the cell change in such a manner that the load caused by possible repeated cell change attempts on the terminal and radio signalling is low.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of copending U.S. patent application Ser. No.11/469,579, filed on Sep. 1, 2006, which is a continuation of U.S. Pat.No. 7,133,383, filed on Mar. 20, 2001, which claims priority to FinnishApplication No. 20000675, filed on Mar. 22, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for controlling cell change in aservice network, in which method a terminal performs neighbour cellmeasurements for cell change, the network makes a decision about thecell change on the basis of the results of said measurements, currentnetwork load and the service needs of the terminal, and the networksends to the terminal a cell change order instructing it to switch overto a new cell. The invention additionally relates to a base station of acellular radio system, which base station comprises means for providingservice and means for receiving signalling messages from a terminal aswell as means for generating signalling messages and sending them toterminals. Furthermore, the invention relates to a terminal of cellularradio system, which terminal is equipped with means for connecting to acertain service and which comprises means for receiving signallingmessages from base stations and means for performing cell specificmeasurements in order to find a suitable serving cell. In addition, theinvention relates to a cellular radio system comprising base stationsand associated cells as well as terminals, wherein the base stations areequipped with means for conveying signaling messages between the basestation and a terminal, and the terminals are equipped so as to operateat a certain service level and to convey signalling messages between theterminal and base station.

2. Brief Description of Related Developments

To provide background for the invention we will next describe someexamples of prior-art solutions for changing the active cell in USM(Global System for Mobile telecommunications) and in the General PacketRadio Service (GPRS) associated with it. FIG. 1 illustrates the basicstructure of a GSM network. The mobile communication network typicallycomprises at least one core network (CN) and one or more radio accessnetworks (RAN). Core networks comprise various switching systems whichmay offer different value-added services in addition to the versatilecommunications features. A core network comprises mobile servicesswitching centres (MSC), other network elements, which in GSM includee.g. the serving GPRS support node (SGSN) and gateway GPRS support node(GGSN), and the associated transmission systems. The radio accessnetworks are located between the core network and the terminals. A radioaccess network comprises base stations (BS) and a radio networkcontroller (RNC). Each base station is connected via a fixed line to theradio network controller. The radio network controllers in turn areconnected via fixed lines to at least one core network node. Between theterminals and core networks there may operate one or more radio accessnetworks, and a terminal may be connected to a plurality of corenetworks via a radio access network.

A terminal of a cellular radio system always attempts to choose a basestation in the coverage area, or cell, of which it camps. In theexemplary situation depicted by FIG. 2, a need for a change of cellsarises at least at points P2 to P7. Conventionally, the cell change hasbeen based on measuring the strength of the received radio signal fromneighbouring cells 1 to 11 either at the terminal or base station. InGSM, for example, each base station broadcasts, at a so-called broadcastfrequency, a signal which has different frequencies for the adjacentbase stations. In GSM, a base station sends to a terminal, on theso-called broadcast control channel (BCCH), the parameters used by theterminal to calculate the so-called C values. Most commonly, theso-called C1 value is used in the calculation. The calculation of the C1value is described in more detail in Michel Mouly, Marie-BernadettePautet, “The GSM System for Mobile Communications”, section 7.1.2.2.

Terminals have to measure the reception levels of the received broadcastsignals in order to be able to calculate the C1 values of the cells. Thecell with the highest C1 value is the most advantageous for the radioconnection. In order to optimize cell changes the network may also sendadditional parameters which make possible to use so-called C2 values. Amore detailed description of the use of these parameters is given inreference [1]. The base stations also send to terminals informationabout the BCCH frequencies used in the adjacent cells so that theterminals know what frequencies they have to listen to in order to findthe BCCH broadcasts of the adjacent cells.

GSM system cells provide users with basic services: audio transmission,slow data transmission, and various short message services. GSMoperation has been standardized by the European TelecommunicationStandardization Institute (ETSI). In GSM, the cells overlap with respectto their coverage areas, so at a cell boundary a decision has to be madeabout which one of the cells the terminal is to use. In addition to thequality of the connection the decision is based on other factors, too,such as the traffic load of the different cells of the network, forinstance. In the active state, the decision about which cell theterminal is to use, as well as the decision about the cell change timingis made by the network, which then informs the terminal. According to anETSI standard, a terminal in the active state cannot by itself make adecision to start using the service of another cell. A terminal in theidle state makes the decision about which cell it belongs to based onthe cells' C1/C2 values which it has calculated. From the terminal'sstandpoint the moment of cell change is then not of great significanceas all the system's cells offer the same basic services and the user ofthe terminal will not notice the cell change.

FIG. 3 shows as an example a flow diagram of prior-art operation in acell change situation in connection with a basic GSM service and GPRSservice. While in the idle/active state 30 the terminal performs, in theexample case depicted by FIG. 3, neighbour cell measurements on thebasis of which the network may estimate 31 the need for a cell change.If the result of the estimation is that a cell change is not needed,operation returns to the initial state 30. If a cell change is neededthe network chooses in step 32 one of the neighbour cells as the newtarget cell and sends to the terminal a cell change order 33. This ordermay be conveyed to the terminal in the basic GSM on the common controlchannel (CCCH) or Slow Dedicated Control Channel (SDCCH) advantageouslyusing a RR_CELL_CHANGE_ORDER message, in which the new serving cell isdescribed in more detail by means of a base station identity code (BSIC)and at least the information about the absolute radio frequency number(ARFCN) of the new cell's base station. If the terminal at the initialstage 30 is connected to a GPRS service on a cell supporting PCCCH, thecell change order is conveyed on the packet common control channel(PCCCH), advantageously using a PACKET_CELL_CHANGE_ORDER message. Havingreceived one of the above-mentioned messages the terminal mustimmediately attempt 34 a cell change if the operation takes place in asystem according to the prior art. If the terminal does not know thetiming information of the new cell, it is not possible to immediatelyget synchronized to the timing used by the new cell, which is indicatedat step 35. In this situation the terminal returns to using the originalcell and informs 37 the network that the cell change was unsuccessfulusing e.g. a PACKET_CELL_CHANGE_FAILURE message if the terminal isconnected to a GPRS service. After that the network may give a new cellchange order to the terminal, possibly targeted at the same cell. In theworst case, there may be several failed cell change attempts and returnsto the original cell before a successful cell change. If, however, theterminal knows the new cell's timing information and is thus able toimmediately synchronize to the cell's timing, it starts to use the newcell already at step 34. Successful step 35 is then followed by theactive/idle state 36.

The prior—an approach described above has many drawbacks. The cellchange fails if the terminal does not know the timing information of thenew cell. If the cell change fails, a service used by the terminal,which is above the basic service, such as e.g. GPRS, may be disconnectedbecause of the failed cell change. In any case the terminal may have tomake several attempts to change cells, which adds to the cell changerelated signalling between the terminal and the base station in theserving cell.

An object of this invention is to provide a cell change procedure whichminimizes unsuccessful cell change attempts to cells with which theoperation of the terminal cannot be immediately synchronized at themoment of receiving the cell change order.

SUMMARY OF THE INVENTION

The objects of the invention are achieved by a procedure in which aterminal constantly monitors the timing information of the neighbourcells known to it and conveys that information to the network if thenetwork orders the terminal to switch over to a cell the timinginformation of which is unknown to the terminal. As soon as the networkbecomes aware of the conflict, it selects on the basis of the neighbourcell information sent by the terminal another cell to which the terminalcan immediately switch over without problems. The network may thenconsequently send to the terminal a new cell change order, instructingit to switch over to a cell with which the terminal has internalsynchronization and thereby can be synchronized without problems.

It is characteristic of the cell change method according to theinvention that if the new serving cell assigned to a terminal in a cellchange order is a cell the timing information of which is unknown to theterminal after neighbour cell measurements, the cell change may not takeplace but instead the terminal may send to the base station a cellchange failure message which may include various information such as thecause of the failure as well as neighbour cell information.

It is characteristic of the base station according to the invention thatit comprises means for receiving and processing information coming froma terminal, which information is adapted so as to convey to the basestation information about those neighbour cells known to the terminalthe internal timing information of which is known to the terminal.

It is characteristic of the terminal according to the invention that itis equipped with means for determining neighbour cell timing informationand means for sending neighbour cell information to the base station ofits current cell when the timing information of the base station of thenew cell assigned in a cell change order to the terminal by the servingbase station is unknown to the terminal.

It is characteristic of the cellular radio system according to theinvention that it comprises information, which is known to a terminal,about a set of neighbour cells of said terminal, the timing informationof the base stations of which neighbour cells the terminal hasdetermined, whereby said system is adapted so as to convey, after a cellchange order addressed to the terminal, said information from theterminal to a base station in a signalling message for the purpose ofselecting a new serving cell if the target cell assigned to the terminalin the cell change order is not included in the set of neighbour cellsdetermined by the terminal.

Advantageous embodiments of the invention are presented in the dependentclaims.

The basic idea of the invention is as follows: A cellular phone isallowed not to make a cell change attempt to a cell the timinginformation of which is unknown to it. This behaviour may be either setas a default in the terminal or it may be enabled in the terminalseparately for each case by means of a message sent to it by thenetwork. In practical operation, a cellular phone measures the neighbourcell information according to the prior art. This way it determines thetransmission frequencies used by the base stations of neighbour cells,as well as the timing information, if possible. Thus it is possible forthe terminal at any given time to know which base stations in thenetwork can be chosen as new serving cells without problems. As a cellchange order comes from the network, the switch can be performed withoutproblems and in accordance with the prior art to a cell the timinginformation of which is known to the terminal and with which it is thuspossible to synchronize without problems. In the case where the switchshould be made to a cell with which terminal has no internal timing, theterminal, instead of attempting a cell change, uses a cell changefailure message according to the invention to inform the network aboutthe cells/base stations with which a cell change would be immediatelysuccessful. At this stage the terminal stays with the original celluntil either the network assigns to it by means of a new cell changeorder another cell with which the terminal can synchronize immediatelyor until other cell change constraints/rules are fulfilled. After that,the cell change takes place as in the prior art.

An advantage of the invention is that the number of failed cell changeattempts is reduced.

Another advantage of the invention is that the load caused by failedcell change attempts on network signalling is reduced.

A further advantage of the invention is that possible service breakscaused by failed cell change attempts are reduced.

A yet further advantage of the invention is that the availability ofcertain services (such as GPRS) is improved as unnecessary failed cellchange attempts are eliminated or reduced and the terminal can bedirectly assigned to a cell that can immediately provide the serviceneeded by the terminal.

DESCRIPTION OF THE DRAWINGS

The invention is described in detail in the following. Reference is madeto the attached drawings in which

FIG. 1 shows by way of example the principal elements of a GSM network,

FIG. 2 shows by way of example a cell change according to the prior artin the GSM system,

FIG. 3 shows by way of example a flow diagram of a cell change accordingto the prior art in the GSM system,

FIG. 4 shows by way of example a flow diagram of a cell change accordingto the invention in the GSM system, and

FIG. 5 shows by way of example the essential structure of a terminalaccording to the invention.

DETAILED DESCRIPTION

FIGS. 1, 2 and 3 were discussed in connection with the description ofthe prior art. FIG. 4 shows by way of example an advantageous method ofrealizing the improved cell change according to the invention. Operationaccording to the invention may be set as a default in a terminal or theterminal may be enabled to operate according to the invention by meansof a special message sent by the network. The cell change starts fromthe active/idle state 40 in which the cell change need is from time totime estimated 41. In the example case depicted by FIG. 4 it is thenetwork that performs said estimation of the cell change need. If thereis no need to change cells, operation returns to step 40. If a cellchange is required the network selects in step 42 one of the possibleneighbour cells as the new target cell. In the active state the networkissues a cell change order in step 43 and in the basic GSM this order isconveyed on the CCCH or SDCCH to the terminal advantageously using aRR_CELL_CHANGE_ORDER message and in GPRS on the PCCCH or on the PACCHadvantageously using a PACKET_CELL_CHANGE_ORDER message. When theterminal has received the order, it will not attempt a cell changeimmediately but finds out in step 44 whether the timing information ofthe new cell is known. If immediate synchronization with the timing ofthe new cell is impossible the terminal transmits advantageously amodified PACKET_CELL_CHANGE_FAILURE message 47 or another message usedin connection with cell change failures. The message in questionadvantageously includes additional information about those neighbourcells to which the terminal can switch over immediately withoutsynchronization problems. Having received the message the network makesa new cell change decision among the cells that were included in themessage 47 from the terminal. The network sends a new cell change order43, and in step 44 it is found that a cell change can be performed, andin step 45 the terminal immediately makes a cell change attempt. Thesuccess of the cell change attempt is tested in the step 45. If the cellchange is not successful the terminal transmits advantageously amodified PACKET_CELL_CHANGE_FAILURE message in the step 47. If the cellchange is successful the operation ends up in the active/idle state 48,and the procedure starts over from the beginning 40.

When a terminal according to the invention sends a message about thefact that it does not know the timing information of the cell assignedto it, the terminal may convey said information advantageously using aCELL_CHANGE_FAILURE, PACKET_CELL_CHANGE_FAILURE or HANDOVER_FAILUREmessage. Into the message the terminal adds information about theneighbour cells the timing information of which the terminal knows andwith which a cell change would be immediately successful. On the basisof this information the network can address to the terminal a new cellchange order in which the target cell is one of the neighbour cellsindicated by the terminal.

Below it is shown by way of example how the permission/informationenabling operation according to the invention can be conveyed to aterminal in a RR_CELL_CHANGE_ORDER or PACKET_CELL_CHANGE_ORDER message.The information may also be conveyed in other messages used in thecommunication between the terminal and the network, such as e.g. systeminformation (SI) and packet system information (PSI) messages. Thefollowing shows by way of example how the additional informationrequired by the invention can be conveyed in the GPRS service to aterminal in a PACKET_CELL_CHANGE_ORDER, as expressed by its informationelements:

<Packet Cell Change Order message content>::= <PAGE_MODE : bit(2) {{0<Global TFI :Global TFI IE>> | 10<TLLI:bit (32)>} {0-Message escape{IMMEDIATE_REL:bit> <ARFCN: bit (10) <BSIC: bit (6) <NC MeasurementParameters:<NC measurement Parameters struct>><Packet_cell_change_order_options: bit (3)> <padding bits> |<Non-distribution part error: bit(*)=<no string>>} | <message escape: 1bit(*) = <no string>>} | Address information part error: bit(*) = <nostring>>} |<Distribution part error: bit(*) = <no string>>;

The message tells, using three bits in the information element“Packet_cell_change_order_options” according to the invention, how theterminal is allowed to operate when it receives a cell change order. Theother information elements comply with the standard. The bitcombinations in the additional information according to the inventionmay be interpreted advantageously according to Table 1, for instance.

TABLE 1 Information element “Packet_cell_change_order_options” accordingto the invention Bits 3 2 1 0 0 0 MS shall obey the cell change order0 0 1 MS shall without cell change reply with PACKET_CELL_CHANGE_FAILURE. . . 1 0 0 MS shall without cell change reply withPACKET_CELL_CHANGE_FAILURE 1 0 1 Reserved . . . 1 1 0 Reserved 1 1 1 MSshall without cell change reply with PACKET_CELL_CHANGE_FAILURE

Bit combination 0-0-0 in Table 1 corresponds to operation according tothe prior art, whereby the terminal shall obey the cell change order ithas received. If one of bit combinations from 0-0-1 to 1-0-0 is assignedto the terminal, the terminal shall not in uncertain cases perform acell change but shall instead send a PACKET_CELL_CHANGE_FAILURE messageto the network. The received bit combination determines the maximumnumber of neighbour cells the information of which shall be included bythe terminal in said reply message. Combination 0-0-1 means that theinformation of one neighbour cell will be sent, combination 0-1-0 meansthat the information of two neighbour cells will be sent, combination0-1-1 means that the information of three neighbour cells will be sent,and combination 1-0-0 means that the information of four neighbour cellswill be sent. Combination 1-1-1 causes the transmission of aPACKET_CELL_CHANGE_FAILURE message that does not include neighbour cellmeasurement data. The other combinations may be used for other cellchange related functions if necessary.

The Packet_cell_change_order_options information element may be shorterthan the three bits mentioned above. In that case the number ofdifferent bit combinations naturally becomes smaller, and the number ofpossible neighbour cells included in the message may advantageously benone for a certain bit combination and e.g. three for a certain secondbit combination.

Below it is shown by way of example how the information according to theinvention can be conveyed from a terminal to the network in aPACKET_CELL_CHANGE_FAILURE or HANDOVER_FAILURE message, expressed astheir information elements, in a situation where the probability of asuccessful cell change is small because the timing information of thenew cell is unknown to the terminal.

<Packet Cell Change Failure message content>::= <TLLI:bit (32)>} <ARFCN:bit (10) <BSIC: bit (6) <Neighbour_cell_reporting: <neighbour cellreporting struct> <spare padding>

The new information element “Neighbour_cell_reporting” according to theinvention may have the same basic structure as that described in a tablepresented in the ETSI document Reference [2]. In the operation accordingto the invention the information element in question has then thestructure illustrated in Table 2:

TABLE 2 “Neighbour_cell_reporting” information element according to theinvention 8 7 6 5 4 3 2 1 CAUSE NO-OF- BCCH- octet 1 NCELLS FREQ- NCELL1  BCCH-FREQ-NCELL 1 BSIC-NCELL 1 octet 2 BSIC- RXLEV-NCELL 1 BCHH octet3 NCELL FREQ 1 NCELL 2 BCCH-FREQ-NCELL 2 BSIC-NCELL 2 octet 4 BSIC-NCELL2 RXLEV-NCELL 2 octet 5 BCCH-FREQ-NCELL 3  BSIC-NCELL 3 octet 6 BSIC-NCELL 3  BCCH-FREQ-NCELL-3 octet 7 RXLEV octet 8 NCELL 3

Table 2 shows a measurement report according to the invention whichreport advantageously comprises eight octets. The first four bits of thefirst octet advantageously indicate the cause of the cell changefailure. Operation according to the invention can possibly use thealready existing cause codes but it is possible to add to them a newcode for the operation according to the invention. The next two bits inthe first octet indicate the number of those neighbour cellsNO-OF-NCELLS in the report the timing information of which is known tothe terminal on the basis of the neighbour cell measurements performedby it. In this example case the data of three neighbour cells areadvantageously transmitted. The last two bits in the first octet areused to convey the frequency information of a first neighbour cellBCCH-FREQ-NCELL 1, and the first three bits in the second octet are alsoused for this same purpose. The frequency information can beadvantageously conveyed through either the absolute radio frequencycarrier number ARFCN the different values of which refer to certainpredetermined frequencies or as indexes to the list of neighbours givenby the network to the terminal. The rest of the bits in the second octetare used to convey part of the neighbour cell identity informationBSIC-NCELL 1, and the rest of that information is conveyed through thefirst bit of the third octet. Next six bits in the third octet are usedto convey the level measurement data of the first neighbour cell RXLEV5NCELL 1. The data for the two other neighbour cells are transmitted inthe following octets in the order described above. The last octet to beused is octet number eight, and its first bit is used to convey the lastbit of the level measurement data of the third neighbour cell RXLEV 3.

In an advantageous embodiment no ARFCN information is transmitted assuch in the measurement report but the ARFCN data in question areindexed in the cellular system in a manner known to both the terminaland the network. In this case it suffices to include the indices inquestion in the measurement report according to the inventiontransmitted to the base station.

In another advantageous embodiment the terminal arranges the neighbourcell data in the measurement report advantageously in the orderaccording to the received RX level (RXLEV) measured by it. In that casethe level measurement data may either be included in the measurementreport according to the invention or they may be left out of it. Byleaving the level measurement data out it is possible to reduce theamount of data transmitted in the cellular system, while still beingable to convey to the network the information about the order which fromthe cell change standpoint is best for the terminal.

FIG. 5 shows in the form of simplified block diagram a terminal 500,advanta-geously a mobile station, according to the invention and itsconnection to a cellular telephone network. The mobile station comprisesan antenna 501 for receiving radio-frequency (RF) signals transmitted bybase stations. A received RF signal is directed by a switch 502 to a RFreceiver 511 where the signal is amplified and converted digital. Thesignal is then detected and demodulated in block 512. Block 513 performsdecryption and deinterleaving. Then follows signal processing in block530. Received data may be stored as such in the mobile station's memory504 or, alternatively, the processed packet data are taken after thesignal processing to a potential external device such as a computer. Acontrol unit controls the above-mentioned reception blocks in accordancewith a program stored in the unit. Transmission from the mobile stationis performed e.g. as follows. Controlled by the control unit 503, block533 performs possible signal processing on the data and block 521performs interleaving and encryption on the processed signal to betransmitted. Bursts are generated from the encoded data, block 522,which are modulated and amplified into a RF signal to be transmitted,block 523. The RF signal to be transmitted is led to the antenna 501through the switch 502. Also these processing and transmission functionsare controlled by the control unit 503. Also illustrated in FIG. 5 are akeypad 531 for inputting information or commands which may be stored inmemory 504. A display 532 can be used to display data stored in thememory 504.

In the terminal of FIG. 5, the parts that are essential from theinvention's perspective include the prior-art reception blocks 511-513,by means of which the terminal receives, demodulates and decodes themessages transmitted by base stations and measures the received signalpower levels, as well as the control unit 503 which processes theinformation contained in the messages and controls the operation of theterminal. Additional features required by the operation according to theinvention have to be added to the operation of the control unit in orderto implement the message indicating cell change failure. Part of thememory 504 of the terminal must be allocated to information relating toneighbour cell selection, such as e.g. information about neighbourcells' BCCH frequencies received from base stations, information aboutthe received signal level, information about the neighbour cells' timinginformation, and information about the quality of service provided bythe neighbour cells. For this purpose it is naturally possible to use adetachable memory device in connection with the terminal, say a SIM cardwhich is known as such. By means of the transmission blocks 521-523 theterminal sends the cell change messages, both those according to thestandards and those according to the invention, to base stations.

The hardware requirements caused by the invention on prior art basestations are minimal. Base station 551 and/or base station controller552, and switching center 553, have access to a database (not shown inFIG. 5) containing neighbour cell information about the BCCH frequenciesand service levels, such as the ability of neighbour cells to provideGPRS service, for example. To this database it is possible to addinformation according to the invention sent by the terminal about thecells for which the terminal indicates that a cell change to them isimpossible at that moment, and also information about the cells to whicha cell change could be successfully performed at that moment. On thebasis of the information received the base station can then assignanother cell to the terminal. The base station 551 is coupled to theterminal 500 via antenna 550.

Naturally the embodiments described above are exemplary only and do notlimit the application of the invention. Particularly it should be notedthat although the examples described above relate to the GSM and GPRSsystems, the invention is applicable to any other digital cellularsystem as well. Especially we could mention the DCS1800 (DigitalCommunications System at 1800 MHz), IS-54 (Interim Standard 54) and PDC(Personal Digital Cellular) and in particular the forthcomingthird-generation digital cellular UMTS (Universal MobileTelecommunications System). In addition, the inventional idea can beapplied in many ways within the limits defined by the claims.

REFERENCES

-   [1] ETSI 05.08 version 6.4.0-   [1] ETSI 04.08 version 6.2.0 section 10.5.2.20 FIG. 10.5.47

1-24. (canceled)
 25. A method comprising: sending, to a terminal, afirst message comprising first identification information for aplurality of target cells and a command to change a communication linkfrom a source cell to a first target cell of the plurality of targetcells; and receiving, from the terminal, a second message indicating afailure to change the communication link, wherein the second messagefurther comprises second identification information for at least onetarget cell of the plurality of target cells with which the terminal issynchronized.
 26. A method as in claim 25, wherein the first messagecomprises a RR_CELL_CHANGE_ORDER message sent on a common controlchannel (CCCH) or a stand-alone dedicated control channel (SDCCH) in aglobal system for mobile communications (GSM) network.
 27. A method asin claim 25, wherein the first message comprises aPACKET_CELL_CHANGE_ORDER message sent on a packet common control channel(PCCCH) or a packet associated control channel (PACCH) in a generalpacket radio services (GPRS) network.
 28. A method as in claim 25,wherein the first message comprises a system information message or apacket system information message.
 29. A method as in claim 25, whereinthe second message comprises a CELL_CHANGE_FAILURE message, aPACKET_CELL_CHANGE_FAILURE message, or a HANDOVER_FAILURE message.
 30. Amethod as in claim 25, wherein the second message further indicates acause of the failure to change the communication link from the sourcecell to the target cell.
 31. A method as in claim 25, wherein the atleast one target cell with which the terminal is synchronized comprisesat least one target cell for which timing information is known.
 32. Amethod as in claim 25, further comprising: using the second informationto select a second target cell from the at least one target cell withwhich the terminal is synchronized; and sending, to the terminal, athird message comprising a second command to change the communicationlink from the source cell to the second target cell.
 33. A computerprogram product comprising program instructions embodied on a tangiblecomputer-readable medium, execution of the program instructions by acontrol unit of an electronic device resulting in operations comprising:sending, to a terminal, a first message comprising first identificationinformation for a plurality of target cells and a command to change acommunication link from a source cell to a first target cell of theplurality of target cells; and receiving, from the terminal, a secondmessage indicating a failure to change the communication link, whereinthe second message further comprises second identification informationfor at least one target cell of the plurality of target cells with whichthe terminal is synchronized.
 34. A computer program product as in claim33, wherein the first message comprises a RR_CELL_CHANGE_ORDER message,a PACKET_CELL_CHANGE_ORDER message, a system information message or apacket system information message.
 35. A computer program product as inclaim 33, wherein the second message comprises a CELL_CHANGE_FAILUREmessage, a PACKET_CELL_CHANGE_FAILURE message, or a HANDOVER_FAILUREmessage.
 36. A computer program product as in claim 33, wherein thesecond message further indicates a cause of the failure to change thecommunication link from the source cell to the target cell.
 37. Acomputer program product as in claim 33, wherein the at least one targetcell with which the terminal is synchronized comprises at least onetarget cell for which timing information is known.
 38. A computerprogram product as in claim 33, execution of the program instructionsresulting in operations further comprising: using the second informationto select a second target cell from the at least one target cell withwhich the terminal is synchronized; and sending, to the terminal, athird message comprising a second command to change the communicationlink from the source cell to the second target cell.
 39. An electronicdevice comprising: a transmitter configurable to send, to a terminal, afirst message comprising first identification information for aplurality of target cells and a command to change a communication linkfrom a source cell to a first target cell of the plurality of targetcells; and a receiver configurable to receive, from the terminal, asecond message indicating a failure to change the communication link,wherein the second message further comprises second identificationinformation for at least one target cell of the plurality of targetcells with which the terminal is synchronized.
 40. An electronic deviceas in claim 39, wherein the first message comprises aRR_CELL_CHANGE_ORDER message, a PACKET_CELL_CHANGE_ORDER message, asystem information message or a packet system information message. 41.An electronic device as in claim 39, wherein the second message furtherindicates a cause of the failure to change the communication link fromthe source cell to the target cell.
 42. An electronic device as in claim39, wherein the at least one target cell with which the terminal issynchronized comprises at least one target cell for which timinginformation is known.
 43. An electronic device as in claim 39, executionof the program instructions resulting in operations further comprising:using the second information to select a second target cell from the atleast one target cell with which the terminal is synchronized; andsending, to the terminal, a third message comprising a second command tochange the communication link from the source cell to the second targetcell.
 44. A method comprising: sending, to a mobile station, a cellre-selection command message comprising first identification informationfor a plurality of target cells and a command to change a communicationlink from a source cell to a first target cell of the plurality oftarget cells, wherein the target cell comprises a global system formobile communications (GSM) cell with which the mobile station is notsynchronized; and receiving, from the mobile station, a packet cellchange failure message indicating a failure to change the communicationlink, wherein the packet cell change failure message further comprisessecond identification information for at least one target cell of theplurality of target cells with which the mobile station is synchronized.45. A method as in claim 44, wherein the packet cell change failuremessage further indicates a cause of the failure to change thecommunication link from the source cell to the target cell.
 46. A methodas in claim 44, further comprising: using the second information toselect a second target cell from the at least one target cell with whichthe mobile station is synchronized; and sending, to the mobile station,a third message comprising a second command to change the communicationlink from the source cell to the second target cell.
 47. A base stationcomprising: a transmitter configurable to send, to a mobile station, acell re-selection command message comprising first identificationinformation for a plurality of target cells and a command to change acommunication link from a source cell to a first target cell of theplurality of target cells, wherein the target cell comprises a globalsystem for mobile communications (GSM) cell with which the mobilestation is not synchronized; and a receiver configurable to receive,from the mobile station, a packet cell change failure message indicatinga failure to change the communication link, wherein the packet cellchange failure message further comprises second identificationinformation for at least one target cell of the plurality of targetcells with which the mobile station is synchronized.
 48. A base stationas in claim 47, wherein the packet cell change failure message furtherindicates a cause of the failure to change the communication link fromthe source cell to the target cell.
 49. A base station as in claim 47,further comprising: a control unit configurable to use the secondinformation to select a second target cell from the at least one targetcell with which the mobile station is synchronized, and wherein thetransmitter is further configurable to send, to the mobile station, athird message comprising a second command to change the communicationlink from the source cell to the second target cell.